1. Field of the Invention
The present invention relates generally to synchronous detectors, and more specifically to synchronous detection for use in a CDMA (code division multiple access) RAKE receiver.
2. Description of the Related Art
According to a prior art synchronous detection scheme used in a CDMA (code division multiple access) RAKE receiver, an input sequence i(n) is cross-correlated with a decision output sequence o(n), and auto-correlation .phi.(n) of the output sequence o(n) is detected. The cross-correlation .theta.(n) is divided by the auto-correlation .phi.(n) and the divided signal Z(n) is supplied to a demodulator where a demodulation of the input sequence proceeds by multiplication with Z(n). The output of the demodulator is combined with the outputs of other synchronous detectors in a combiner. From the output of the combiner, the decision output sequence is produced by a decision circuit. The amplitude of the signal received by each synchronous detector of the RAKE receiver is estimated by performing a square-and-sum process on the input sequence i(n). In addition, the decision error is estimated by calculating e(n)=i(n)-Z(n).o(n) at symbol intervals and a square-and-sum process is performed on the estimated error e(n). According to these estimated amplitude and error, the outputs of the synchronous detectors are weighted so that they are maximal-ratio combined in the combiner.
However, the prior art error estimation requires, at symbol intervals, two multiplication processes at the level of signal e(n), and one subtraction process and two multiplication processes at the level of complex values. Since the CDMA RAKE receiver requires such computation for each of propagation paths, the amount of total computations is substantial.